Yamaha SR250 Power and Temperature Testing

As some of you may know, I am building a 'standard' style (going for some timeless style) Yamaha SR250 and using it as the basis for designing and testing a range of styling and performance parts. I felt that the infamous SR250 was grossly neglected on the aftermarket and thought I could step up and make use of my design/engineering background to make some cool parts for garage builders. Needless to say, I have a pretty strong passion for these little bikes!

Anyway, I decided to start this separate thread for the testing alone so not to detract from the build itself in the original thread. Also because I bought another SR250, bone stock, just for this purpose - to test the performance parts and to test fit the styling parts. I am hoping to eventually make a series of instructional installation videos for the parts on youtube as well - I myself find it easiest to see people do stuff and do what they do, rather than reading instructions ;) Damn left brain!

I intend to be open and honest and unbiased with the process, hoping that if anyone else were to perform the same tests, they would get the same or similar results. Maybe not the smartest business move, but who wants to buy parts that dont work?!

Check out this poor little (albeit ugly af) test mule ;D


I thought I could start by establishing some base lines.

Many people might expect that the SR250 puts out either 17 or 20 horse power - according to Yamaha and many online documents. As with many motorcycle companies, they tend to publish power output at the engine, rather than at the wheel - making the power figure sound much higher because it hasn't had any drive train losses.

I have found two independent period tests from two different magazines (one in Australia and one in Spain) that put rear wheel horse power at a tad over 13. Sorry to burst anyone's bubble here! That figure is a little underwhelming haha. Power at the engine was however at 18 - only two less than Yamaha's claimed 20 in this case.

With regards to head and oil temperature, and oil pressure, I have not been able to find any information online specifically about the SR250. If anyone else has, please share! This means I'll have to try and establish some baselines of my own with the sensors I have installed and get the results from the stock bike first.


First things first were to get the bike running properly and road legal in Sweden. There were quite a few unpleasant surprises when I started working on the bike...

The stock exhaust (as with many of these bikes) had rusted out leaving a hole - so that had to be replaced. Fine. However, when taking the exhaust off to replace it, I discovered that it was attached to the head with one normal M6 bolt, great, then the other, a f*cking wood screw! What?! I was fuming. Some muppet had snapped off the original bolt in the head, then tried to drill it out, drilled waaay crooked, missed the original bolt and ended up with another almost sideways hole in the head. Then the hole was too mashed to tap properly so they used this wood screw. Awesome. I myself have messed up stuff like this, but when you do, it is important to do the repair properly. Easy thing to break off a stuck bolt by mistake. But the bolt should be removed properly, then a thread repair/helicoil set put in its place.

A few other items... had to replace the completely worn drive package, the speedo (faulty), the rear brake light switch, the rear wheel drive studs were stripped, rear tank mount was missing (due to mounting hole being stripped), sump plug was stripped (the hex part, not the thread thank god), the left side cover was held to the bike with two wood screws straight into the air box! If that doesn't fuck up carb settings I dont know what would. Fuel petcock seals shot, pissing fuel all over the ground when disconnecting the tank, oil super black (hadn't been changed in eternity), valve tappets super loose, no gasket used on the alternator side engine cover, three holes in the carb diaphragm (no wonder it was topping out at 95 on the highway), completely stuck air screw and rust particles in the float bowl. I ended up just replacing the whole carb with a spare one I had.

I got to it and replaced/repaired most things. I shrugged everything off apart from the wood screw. Just couldn't let it go. Anyway, I managed to clean up the hole and re-tap it with an M6 tap and got about 6mm worth of decent thread - just enough (I hope) to hold it in place on that side. I think I will some day soon make a big bore kit for these bikes and then I'll be taking the head off anyway and will get it repaired properly.


I also swapped out the silly semi-ape bars that are so uncomfortable for me being 6' 1''. I put some lower profile ones on and now its much more comfortable to ride. When changing bars on the SR250 you end up with copious amounts of extra cable length - black spaghetti everywhere! So I took the opportunity to test fit the short cable kit I am working on for these bikes. They fit pretty well for first samples. I wrote up some notes/revisions and hopefully the next samples will be spot on - ready for a production order.



New Member
Good writeup! This project is really really interesting, keep it coming dude!

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I ended up getting the bike to run really nice after the full service. Pretty much a brand new stock exhaust from my other project, new re-furbed carb I had lying around, oil and filter change, valve tappets set, cam chain set, etc etc. Now it almost pulls 100 merging onto the highway in third! These bikes love revs haha. It cruises pretty comfortable around 110 but struggles a bit after that. It can do that 110 in 4th or 5th though! 5th just runs out of steam a bit on a hill or head wind so you just chop it down and ring it out some more. Makes me realise that perhaps the other SR250s I have owned I haven't rung out hard enough ;D

Once I got it to that level I took it in for inspection and it passed with flying colours. Yay! Now it is registered and insured in Sweden with a sweet Swedish plate!
I also installed the sensors I have been talking about for months! One neat head temp sensor from trail tech - just bolts on with the spark plug (remove crush washer and replace with their copper sensor ring) and has a handy foam tape backing. One oil temp sensor/probe tapped into the sump plug to give me a really nice accurate oil temp. Then one more tapped into the clutch side crank case - right after the oils journey up through the oil filter. The idea is to be able to get a temperature comparison between these two pints. And hopefully establish a correlation between oil and head temp, if there is any in this case. All seem to work great! I did a semi decent wire harness for the sensors so they handle all the testing. I soldered in a couple of alligator clips so I can just connect and disconnect straight to the battery without messing up any of the stock wiring. Then its also easy to take it off later down the track when testing is done. The gauges themselves are held on by an M4 bolt that I drilled and tapped into one of the handle bar mount bolts!

I installed one of these oil filler cap temperature sensors to see how accurate they really are ;)


Well I have done some quick test rides with the sensors and they seem to work pretty well. The one installed after the oil filter seems to cut out after a while and completely dies on the highway. At first I thought it could be because its so exposed - sitting out in the open air and chilling right down to 40C which is the lowest point the gauge will show. That is why I tried to insulate it with exhaust heat wrap. Stupid me! When that didn't help, I tried wiggling the sensor wire at 100kph haha. That seemed to get some flashes of temperature readings. So I thought it was the connection... Re-did that to no avail. Then I thought it might be a faulty sensor... But why does it read correct sometimes? Figured it out... The sensor relies on getting its earth through the connection to the engine cases. I figured this out after looking at the wiring diagram again. The crank case cover perhaps does not have the best connect to the engine - due to the gasket. Then with this bike, the earth is connected to the frame rather than the engine, so perhaps that contributed as well. Anyway, I rigged up a second earth straight to the battery and that seemed to solve it! Happy testing days ahead ;D


I've only been for very short rides so haven't really got it up to full operating temp properly yet but the gauges are pretty cool - especially the head one which has a max reading that resets ever few hours - so you get a max reading for each separate ride.

What is really interesting is that the oil looses between 9 and 12 degrees from its journey from the sump to the oil filter... Always. You can see this in the above picture. I guess the entire engine/crankcases acts like a huge aluminium heat soak. Kinda cool. I double checked this by swapping the sensor plugs so the gauges were connected to the opposite sensors and I got the same readings.
At the same time I bought the temp gauges/sensors, I also bought an oil pressure gauge. An automotive one. How foolish. These engines have roller bearing cranks and cams and therefor operate on very low oil pressure but with a high flow rate. I knew this too. Plain bearing cranks, like in cars and many multi cylinder bikes require A LOT more oil pressure. Therefor, an automotive oil pressure sensor and gauge like the one I had doesn't read anything below 10psi! Because that would be dangerous for any such engine. I realised that installing this would be pretty pointless because I have seen XS650s run anywhere between 4-7psi and all the way down to 1-1.5psi when hot. The same with Harleys apparently. Thats why they call them worry gauges and advice people not to install them - cos you just end up getting distracted and worried looking at it while riding in the heat.

Long story. Anyway, I bought a proper low pressure gauge that will read from 1-15psi and I am hoping to install this instead and get some readings - hot, cold, low rpm, high rpm etc. In the photo you can see the backup earth I installed on the temp sensor as well.


Got out on the test bike over the weekend and recorded some temperatures. It takes quite a while for the bike to get up to temperature, which I guess would be the same for most air-cooled bikes. Outside temp was 18C. Head temp on a 100kph highway peaked at 198C while oil temp got to 95C in the sump and something like 83C at the filter.

The cheap oil filler cap temp gauge was sitting on just a touch over 80C - which is pretty close to what the oil was after the filter - and perhaps the general temp of the crankcases. Not so bad after all!

I have been speaking to a local bike shop with a dyno and will hopefully test some of the performance parts in the second week of June when he has a little more spare time. Sweet!


Before I go into the next massive post I will write, I thought it was important to mention oil... When I did the service on the bike I changed the oil (obviously) and filled up up with some new fancy shit - you could say that I got 'sold' it by one of the local mechanics. It is this Fuchs Silkolene Comp 4 10W-40. Just as a side note, the Yamaha manual recommends SAE 20W/40 type 'SE'. In the past I have used this and have actually tried several others (which I cannot remember) based on recommendations, all of which worked fine.

Anyway, this new oil makes gear shifting really nice - it shifts smoother and more precise than it did before (maybe not saying much cos the oil was nearly black anyway) and the bike seems to run real nice. However, as many have discussed for long periods of time about synthetics in an old bike... the clutch now does slip from time to time right on the power band when shifting sometimes - right around 7000rpm where the most power is. There could be a few reasons for this, one, and perhaps the most obvious - its the damn oil - it just doesn't work with old bikes - something with the clutch plates needing something that the old mineral oils provide. Yep, could be as simple as that. Or, another reason could be that this problem may have been there before and I just didn't notice it because the engine was running pretty rough and wouldn't quite make enough power to do it. OR, the clutch plates were/are actually worn and this new oil with friction modifiers etc just highlights the issue much more than a mineral oil would. I have read this in a few different forums as well. Either way, I think I like the oil and I will replace the clutch plates and springs with the next oil change and see how that works out. They'll certainly need to be stronger if the bike is going to make any more power than stock ;)



New Member
Put the clutch plates from an xt250 in there. A bit thinner steel plates but more of them. Nice grab and shift.
cosworth said:
Put the clutch plates from an xt250 in there. A bit thinner steel plates but more of them. Nice grab and shift.
Cheers for the tip man! Will do that. So it goes from an 11 plate set up to 13? 7 friction plates, 6 clutch plates?
Day 1 of proper testing done. I now realise how hard it will be to get any comparable results - ambient temp, weather, wind speed and riding style all play such a huge part in the running temps of the engine - obviously. So it means that to measure and compare the effect of a performance cooling part will be pretty inaccurate. I need to create a more controlled test environment - like scientists do haha. For example, maybe the bike should be stationary, indoors (with exhaust extraction), fixed temp, fixed to rev at a certain rpm, with a fan blowing on it fixed at a certain setting, for a certain amount of time - then record the results. Then repeat the tests for each different part.

Here is a photo of me trying to capture the temp displayed on this time/temp sign near where my workshop is. Damn camera couldn't pick it up but it did say 26 degrees C.


And here are the results:

Riding on country roads with posted speed limits of 70-90kph, in 4th gear, with the engine running between 4000-7000rpm the engine had read temperatures ranging from:

Head: 165-195C
Oil-Sump: 85-95C
Oil-At Filter: 80-86C

When stopped, I quickly took temp laser gun readings of:

Exhaust Tappet Cover (side): 116C
Exhaust Tappet Cover (front): 109C
Intake Tappet Cover (front): 106C
Oil Filter Cover: 70C

And the cheapish oil filler cap gauge read 78C - which is pretty close to the temp readings at the oil filter.

Then I did two 20K (15mins roughly) there and back runs on the highway. On the way there the bike was running between 110-115kph at 6500-7200rpm (roughly) with a head wind, on the way back with the tailwind it was running at 115-120kph at 7500-7800rpm (roughly). When observing the gauges, the engine had read temperatures ranging from:

Head: 206-214C
Oil-Sump: 105-116C
Oil-At Filter: 92-98C

When stopped, I quickly took temp laser gun readings of:

Exhaust Tappet Cover (side): 142C
Exhaust Tappet Cover (front): 143C
Intake Tappet Cover (front): 130C
Oil Filter Cover: 81C

And the cheapish oil filler cap gauge read 95C - which is pretty close to the temp readings at the oil filter.


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